The principal objectives of this study are: a) to understand the regulation and coordination of synthesis of the individual nonprotein components (phospholipids, fatty acids, steroids) of membranes; b) to investigate the biochemical role or function that these lipid and steroid components have in the membrane; c) to describe the kinetics and site of plasma membrane growth: d) to determine the lipid and steroid composition of the various membranous organelles (mitochondria, etc.) of the cell. To perform these studies, we will employ a unique collection of membrane mutants, some of which do not exist in any other organism. These mutants are the two choline requiring strains, a saturated fatty acid requirer, and an ergosterol deficient strain. The choline mutants will be used to study the physiological effects of variation in the content of lecithin and the mode of regulation of lecithin synthesis. The fatty acid requirer and the ergosterol deficient strain will be employed in a similar fashion. In addition, we will screen some of the existing Ts lethal mutants and some morphological mutants for possible blocks in cardiolipin synthesis. Aspects of this work that are of medical interest deal with studies on contact inhibition, Refsum's disease, and the lecithin deficiencies which produce respiratory deficiency syndrome.